1. Field of the Invention
The present invention relates generally to handoff between heterogeneous networks, called vertical handoff, and in particular, to an apparatus and method for determining a target network for vertical handoff, taking into account the load of each communication system in order to overcome resource imbalance between heterogeneous networks such as a Wireless Local Area Network (WLAN) and an Institute of Electrical and Electronics Engineers (IEEE) 802.16 broadband wireless communication network.
2. Description of the Related Art
With the recent advent of the wireless multimedia era, demands for transmitting a large amount of data on radio channels at high data rates have been increasing drastically. Accordingly, a wireless high-speed data transmission system to support Internet service on mobile channels and radio channels is being developed.
The existing 3rd Generation (3G) communication systems support data rates up to 2 Mbps in a fixed or stationary environment, and 4th Generation (4G) communication systems are intended to provide services with a variety of Quality of Service (QoS) levels to users, aiming at about 100 Mbps. In this context, efforts are being made to develop a WLAN (Wireless Local-Area Network) that can ensure high data rates (ranging from 20 to 50 Mbps) and a new communication system which can offer a guarantee of mobility and QoS for high-speed data service to be provided by 4G communication systems.
In 1999, a WLAN standard (WLAN) was developed in compliance with IEEE 802.11 for short-range communications among stationary users in a limited area such as indoors, densely populated areas, libraries, etc. in an effort to expand the conventional wired LAN. According to technical standards, the WLAN standard provide Internet connectivity at a high rates from between about 11 and 54 Mbps in “hot spot” areas.
WLANs offer benefits of the low cost and easy deployment of Access Points (APs) which are required for installing the hot spots.
However, since WLANs typically have a radius of about 100 m, a large number of APs are needed to cover a broad area. Moreover, handoff between WLANs (i.e., horizontal handoff) is not supported and service provisioning is difficult during roaming within a hot spot.
On the other hand, the IEEE 802.16 system enables seamless, high-speed wireless Internet access through a portable Internet terminal irrespective of time and place in a stationary state or during walking or movement at an intermediate speed (up to 60 km/h). Particularly the IEEE 802.16 system supports data rates of 1 Mbps or higher at which a variety of ultra fast wireless multimedia services can actively be provided, and supports diverse multimedia terminals such as a laptop computer, a Personal Digital Assistant (PDA), and a smart phone, as well.
The IEEE 802.16 system has a cell radius of about 10 km and nationwide service is possible through proper network configurations. Moreover, the IEEE 802.16 system supports handoff and thus provides a seamless service even during fast movement. However, it also has the distinctive shortcoming that there are limitations in satisfying user demands for servicing a large volume of multimedia contents due to low rates of hundreds of kilo bits per second relative to the data rates of the WLAN. Hence, the IEEE 802.16 system is assumed to be a wireless Internet technology suitable for providing a service like the Internet service to users moving at intermediate/low speeds.
As described above, the WLAN and the IEEE 802.16 system have opposite characteristics. Hence, in order to increase system efficiency, there is a desire to provide vertical handoffs between the WLAN and the IEEE 802.16 system according to the circumstances of a user. While handoff generically refers to handoffs between homogeneous networks, i.e., horizontal handoffs, vertical handoffs will also be supported in 4G.
FIG. 1 is a block diagram illustrating typical horizontal and vertical handoffs.
Referring to FIG. 1, Layers 101, 103, and 105 each depict a homogeneous network. Horizontal handoffs 115 and 117 and 119 are illustrated within homogeneous networks 103s and 105, respectively. Vertical handoffs are depicted between heterogeneous networks in steps 107, 109, 111 and 113.
The horizontal handoff occurs between homogeneous networks, i.e., in the same network service area. On the other hand, the vertical handoff is implemented from a small service area to a large service area (e.g., see, steps 107 and 109), or vice versa (e.g., see, steps 111 and 113).
Compared to the horizontal handoff, the vertical handoff between different heterogeneous networks (e.g., the WLAN and IEEE 802.16 systems) should be implemented according to a user situation, taking into account the characteristics of the networks involved. For example, if a user is located within both the service areas of the WLAN and the IEEE 802.16 system and attempts to associate only with the WLAN, seeking high data rate, load imbalance can follow. As more and more users concentrate on the WLAN, the WLAN has deteriorated throughput relative to the IEEE 802.16 system, and thus the IEEE 802.16 system wastes resources.
The WLAN defines two Medium Access Control (MAC) protocols to efficiently share limited IEEE 802.11-compliant radio channels among nodes (stations or terminals): Point Coordination Function (PCF) and Distributed Coordination Function (DCF).
FIG. 2 is a diagram illustrating a MAC protocol for efficiently controlling a plurality of nodes in a typical WLAN.
Referring to FIG. 2, a PCF (Point Coordination Function) 203 runs based on a DCF (Distributed Coordination Function) 201. The DCF 201 is a Carrier-Sense Multiple Access/Collision Avoidance (CSMA/CA)-based protocol. The DCF 201 enables nodes to form a network through distributed contention, rather than controlling channel access by an infrastructure such as APs. The PCF 203 controls channel access by particular stations such as APs on the basis of the DCF 201.
In other words, the DCF 201 supports best-effort services, whereas the PCF 203 implements time-bound services using a threshold to guarantee QoS. Yet, complexity and other obstacles hinder commercial use of the PCF 203 and only the DCF 201 is commercialized in the form of a WLAN card. Therefore, the following description is made in the context of the IEEE 802.11 DCF protocol.
Due to its simplicity and flexibility, the IEEE 802.11 DCF protocol finds wide use in WLAN cards. However, since the DCF runs based on CSMA/CA in a distributed fashion for each node, in case where a large number of nodes are connected to one AP, the packet collision probability between the nodes increases. As a consequence, the throughput of the WLAN decreases, as noted from a graph illustrating changes in throughput with respect of the number of nodes in the WLAN in FIG. 3.
Further regarding the IEEE 802.11 protocol in relation to the WLAN throughput, is support of multiple rates in the physical layer. This implies that each node varies its data rate according to channel condition rather than being fixed at a particular data rate.
Table 1 lists modulation schemes used in a WLAN compliant with IEEE 802.11a/h standards.
TABLE 1ModeModulationCode RateData RateBpS1BPSK1/2 6 Mbps32BPSK3/4 9 Mbps4.53QPSK1/212 Mbps64QPSK3/418 Mbps9516-QAM1/224 Mbps12616-QAM3/436 Mbps18764-QAM2/348 Mbps24864-QAM3/454 Mbps27
Referring to Table 1, available data rates range from 6 Mbps to 54 Mbps through different modulation schemes and code are available in the WLAN.
The reason for using different data rates by different modulation schemes is a variable channel condition arising from the distance between an AP and a node or interference from neighbor nodes. For a node at a good channel condition, a higher-order modulation is used, such as 64-QAM (Quadrature Amplitude Modulation) or 16-QAM in view of a low Bit Error Rate (BER). On the other hand, the use of 64-QAM or 16-QAM increases the BER too much, leading to transmission failure, for a node having a bad channel condition. Therefore, a modulation scheme robust against noise and interference, such as BPSK (Binary Phase Shift Keying) or QPSK (Quadrature Phase Shift Keying) is used, which leads to low data rate.
As described above, because each node uses a different modulation scheme according to its channel condition, the co-existence of nodes with different data rates in the WLAN results in performance anomaly.
FIGS. 4A and 4B are block diagrams illustrating a performance anomaly encountered with the typical WLAN. It is assumed herein that only two nodes exist in the IEEE 802.11b-compliant WLAN.
In the illustrated case of FIG. 4A, two nodes 401 and 403 (i.e., node A and node B, respectively) operate at 11 Mbp. Assuming that node A and node B alternately send data based on contention according to the DCF protocol, the total throughput of the WLAN is 3.3 Mbps per node, considering protocol overhead.
In the illustrated case of FIG. 4B, two nodes 405 and 407 (i.e., node C and node D, respectively) operate at 1 Mbp and 11 Mbps, respectively. Assuming that node C and node D alternately send data based on contention according to the DCF protocol, the total throughput of the WLAN is about 0.76 Mbps per node, considering protocol overhead.
This dropped throughput phenomenon caused by the participation of a low-rate node with high-rate nodes, is called performance anomaly.
Meanwhile, IEEE 802.16 services are classified into real-time service having a QoS requirement intolerant to transmission delay such as video streaming, audio streaming, interactive gaming, etc., and delay-tolerant non-real-time service such as file transfer, multimedia mail, chatting, and electronic trade, and best-effort service tolerant to transmission delay and having no guarantee of resources during a service, such as web browsing and e-mail.
Table 2 below lists significant schemes and requirements for wireless connection of 2.3 GHz portable Internet.
TABLE 2ItemScheme or ValueMultiplexingTDD (Time Division Duplex)Multiple AccessOFDMA (Orthogonal Frequency Division MultipleAccess)Channel10 MHzBandwidthData RateUplink minimum/maximum rate: 128 Kbps/1 Mbpsper SubscriberDownlink minimum/maximum rate: 512 Kbps/3 MbpsFrequency1Reuse FactorFrequencyMaximum frequency efficiency: downlink/uplink (6/1)EfficiencyAverage frequency efficiency: downlink/uplink (2/1)HandoffInter-cell handoff in BS, inter-BS handoff, inter-frequency handoff: 150 ms or belowMobilityMaximum 60 Km/hService CoveragePicocell: 100 mMicrocell: 400 mMacrocell: 1 Km
Table 2 illustrates that the IEEE 802.16 system can allocate resources to nodes in TDD-OFDMA. Therefore, the performance anomaly causing a throughput decrease as encountered with the WLAN does not exist. Consequently, when the number of users increase, the IEEE 802.16 system divides/allocates resources according to the number of the users without decreasing the total throughput.
The WLAN and the IEEE 802.16 system show different throughput changes with respect to the increase in the number of users. Especially, the WLAN may suffer a rapid throughput decrease due to performance anomaly when a low-rate node is added. If vertical handoff is implemented between the WLAN and the IEEE 802.16 system with no regard to the characteristics of both networks in relation to the increase of the number of users, the nodes are highly inclined to move to the WLAN. The resulting decrease in the WLAN throughput causes resources dissipation in the IEEE 802.16 system.
Accordingly, there exists a need for developing a method of increasing the overall efficiency of resources without causing load imbalance between systems.